Adaptation for Childbirth

The majority of women have a gynaecoid pelvis, as oppose to the male android pelvis. The slight differences in their structures creates a greater pelvic outlet, adapted to aid the process of childbirth. When comparing the two, the gynaecoid pelvis has:

A wider and broader structure yet it is lighter in weight

An oval-shaped inlet compared with the heart-shaped android pelvis.

Less prominent ischial spines, allowing for a greater bispinous diameter

A greater angled sub-pubic arch, more than 80-90 degrees.

A sacrum which is shorter, more curved and with a less pronounced sacral promontory.

In addition to the bony adaptations, the sacrotuberous and sacrospinous ligaments can stretch under the influence of progesterone and increase the size of the outlet further.

Clinical Relevance: Assessment of the Female Bony Pelvis

The lesser pelvis is the bony canal through which the fetus has to pass during childbirth. It is therefore of great importance to determine the diameter of this canal and therefore the childbearing capacity of the mother.

The diameter can be determined by a pelvic examination or radiographically. There are two measurements that are of importance:

Obstetric Conjugate

In order to determine the narrowest fixed distance that the foetus would have to negotiate, the minimum antero-posterior diameter of the pelvic inlet is measured.

This distance is between the sacral promontory and the midpoint of the pubic symphysis (where the pubic bone is thickest) and is known as the obstetric conjugate (or true conjugate). However, this measurement cannot be taken clinically, due to the presence of the bladder.

Diagonal Conjugate

The diagonal conjugate is the alternative, measuring from the inferior border of the pubic symphysis to the sacral promontory and can be measured manually via the vagina.

(To do this you use the tip of your middle finger to measure the sacral promontory and then using the other hand to mark the level of the inferior margin of the pubic symphysis on the examining hand. You then use the distance between the index finger and the pubic symphysis to measure the obstetric conjugate, ideally 11cm or greater)

In addition to measuring the diagonal conjugate, a mid-pelvis check is carried out. Here, the clinician is testing for straight side walls and measuring the bispinous diameter which is narrowest part of the pelvic canal. The width of the subpubic angle at the pelvic outlet can be determined by the distance between the ischial tuberosities.

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